Build: Add CSS files in /public/sections to the watched hot-reload files list

[ryelle]

After #18823 was merged, I noticed that hot-reloading CSS wasn't working in the Store extension anymore. I traced this to a list of watched CSS files, publicCssFiles, in /server/bundler/css-hot-reload.js. The very simple solution I've come to here is just adding the files in /public/sections to this list as well.

There are a few things I noticed, but I don't think they're necessarily problems:

• It always reloads style-debug.css, even though I'm pretty sure nothing's changed there
• It reloads section files regardless of whether you're on that section— if I make a change in the store extension CSS, it reloads onto any calypso.localhost, even if I'm not on a store page.

This still doesn't watch the files in public/sections-rtl, and at this point I'd suggest either something like recursive-readdir-sync, or looping over an array whitelist of PUBLIC_DIRs.

To test

• Start calypso: npm start
• Load up a store page, ex the dashboard: /store/:site (where site is an atomic site)
• Make a change in store CSS (extensions/woocommerce/style.scss)
• Open the console and watch for Building CSS…, Reloading CSS… <…woocommerce.css
• See your changes 🎉

[matticbot]

Test live: https://calypso.live/?branch=fix/sections-hot-reloading

[allendav]

Works very nicely.

[coderkevin]

LGTM, tests good!